1. Field of the Invention
The present invention relates to silicon carbide whisker reinforced ceramic composites.
2. Description of the Prior Art
Silicon carbide (SiC) whisker reinforced ceramic composites have generated significant interest due to their improved fracture toughness, strength, and other mechanical properties. Commercial applications for these materials have been slow to materialize, however, because of the difficulties associated with developing efficient and economical methods of manufacturing the composites.
One of the primary difficulties facing the industry concerns the inherent inhibition of densification of the composite when whiskers are added to the matrix material. Accordingly, most studies reported in the literature utilize hot-pressing to achieve high densities [Becher et al, J. Am. Ceram. Soc., Vol. 63[12], pp. C-267-C-269 (1984); Wei et al, Am. Ceram. Soc. Bull., Vol. 64[2], pp. 298-304 (1985); Gadkaree et al, Am. Ceram. Soc. Bull., Vol. 65[2], pp. 370-376 (1986); Gac et al, J. Am. Ceram. Soc., Vol. 68[8], pp. C-200-C201 (1985); Chokshi et al, J. Am. Ceram. Soc., Vol. 68[6], pp. C-144-C-145 (1985); Shalek et al, Am. Ceram. Soc. Bull., Vol. 65[2], pp. 351-356 (1986); Samanta et al, Ceram. Eng. Sci. Proc., Vol. 6[7-8], pp. 663-672 (1985); Homeny et al, Am. Ceram. Soc. Bull., Vol. 66[2], pp. 333-338 (1987); Buljan et al, Am. Ceram. Soc. Bull., Vol. 66[2], pp. 347-352 (1987); Porter et al, Am. Ceram. Soc. Bull., Vol. 66[2], pp. 343-347 (1987)].
Hot pressing, however, renders it extremely difficult to achieve control of the shapes and sizes of the ultimate ceramic article. Moreover, hot pressing techniques are notoriously cost-inefficient and time-consuming.
Recently, considerable attention has been paid to pressureless sintering techniques [Tiegs et al Am. Ceram. Soc. Bull., Vol. 66[2], pp. 339-342 (1987); Takao et al, pp. 699-708 in Ceramic Powder Science, Advances in Ceramics, Vol. 21, Eds. Messing et al, American Ceramic Society, Westerville, Ohio (1987); Barclay et al, J. Mat. Sci., Vol. 22, pp. 4403-4406 (1987); Sacks et al, J. Am. Ceramic. Soc., Vol. 71[5], pp. 370-379 (1988); Sacks et al, Ceram. Eng. Sci. Proc., Vol. 9[7-8], pp. 741-754 (1988); Sacks et al, pp. 440-451, Ceramic Powder Science II, A, Ceramic Transactions, Vol. 1, Eds. Messing et al, American Cer. Soc., Westerville, Ohio (1988); Sacks et al, pp. 175-184 in High Temperature/High Performance Composites, Mat. Res. Soc. Symp. Proc., Vol. 120, Eds., F.D. Lemkey et al, Materials Research Society, Pittsburgh, Pa. 1988.] Several approaches have been employed to enhance densification, including (1) using additives which promote liquid-phase sintering, (2) using whiskers with lower aspect ratios in order to reduce the tendency to form shrinkage-inhibiting whisker-network structures and (3) preparing compacts with improved green microstructural characteristics, e.g., higher green density, absence of agglomerates, etc.
U.S. Pat. Nos. 4,652,413; 4,774,209; 4,657,877 and 4,746,635 describe recent attempts to solve the problems inherent in achieving commercially attractive SiC whisker-reinforced ceramic composites; however, all are subject to one or more serious disadvantages. A primary disadvantage resides in the fact that it is virtually impossible, following conventional pressureless sintering techniques, to achieve anything approaching maximum relative densities, especially at high SiC loadings.
It is an object of the present invention to provide novel SiC whisker ceramic composites and improved methods for their manufacture which are not subject to the disadvantages attendant the prior art methods.